FIG. 8 is a block diagram showing the construction of a conventional semiconductor laser module. The conventional semiconductor laser module includes a laser diode (LD) 1, an optical fiber 2 for transmitting the light emitted from the front side of the laser diode 1, and a lens 3 disposed between the LD 1 and the optical fiber 2 for converging the light emitted from the front side of the laser diode 1. Also, a monitor photodiode (PD) 4 is disposed on the rear side of the LD 1 to monitor the light emitted from the rear side of the LD 1, and an APC (automatic power control) circuit 5 is used for controlling the driving current of the LD so as to make a monitor PD current constant to keep output of the optical fiber constant.
Conventionally, in semiconductor laser modules for dealing with a short wavelength band around 980 nm, Si—PD having a high sensitivity concerned above wavelength band have been used as a monitor PD.
FIG. 9 shows a relationship between LD operating current and fiber end optical output of a semiconductor laser module of 980 nm band using Si—PD. In FIG. 9, optical output from the front side due to temperature of the LD is changed to 72.9 mW at 5° C. and 66.1 mW at 45° C. when an operating current is 150 mA, that is, a reduction of 0.43 dB occurs (=10×LOG (66.1/72.9)).
FIG. 10 shows measurement results of sensitivity characteristic of Si—PD relative to temperature. Data in FIG. 10 indicates an increase of 0.25 to 0.3 dB between 5° C. and 45° C. That is, the sensitivity characteristic of Si—PD presents a positive temperature coefficient, at which the sensitivity is enhanced as temperature rises.
FIG. 11 shows a relationship between monitor PD current and fiber end optical output of a semiconductor laser module of 980 nm band using an Si—PD. When a monitor operating current of Si—PD is 0.116 mA, the optical output is 72.9 mW at 5° C. and 62.8 mW at 45° C., that is, a reduction of 0.65 dB (=10×LOG (62.8/72.9)) occurs. This indicates a tracking error of a semiconductor laser module, and means a decline above temperature change of the LD as apparent from comparison with FIG. 9.
Reasons for the tracking error in a semiconductor laser module include a change in reflection coefficients of an LD on front and rear sides due to temperature, deviation of optical axis of the optical fiber. Usually, the reflection coefficient of the LD is as low as several percents on the front side but is as high as 90% or higher on the rear side, whereby change is large due to temperature in optical output from the front side of an LD but is little on the rear side of an LD. Therefore, in a case of performing an APC control with a constant monitor PD current, a large amount of tracking error occurs due to temperature change. That is, a decline in optical output from the front side in FIG. 9 substantially corresponds to that. However, in the case of using Si—PD, the PD sensitivity is enhanced by temperature rise as shown in FIG. 10, so that even upon reception of the same optical output the monitor PD current becomes large with temperature rise. This current is fed back to an APC circuit in which the monitor PD current is controlled to be made constant, whereby the APC circuit operates in a direction of decreasing an LD operating current (direction of decreasing an optical output) to thereby make a tracking error further larger.
As described above, conventional semiconductor laser modules of 980 nm band use an Si—PD for APC circuits, and so a tracking error due to temperature change becomes larger than that due to an LD itself.